Xcode: Setting GCC_PREPROCESSOR_DEFINITIONS for different build configurations?
Make sure there isn't a setting called "Preprocessor Macros". This is the same as GCC_PREPROCESSOR_DEFINITIONS.
OTHER_CFLAGS if I understood the question right. And pass your defines like -DDEBUG or something.
And yes, if you need to assign a value to a macro, you could do something like this - '-DSOME_MESSAGE=@"Hello, World!"'.Note ' symbols around - they are required. SOME_MESSAGE macro will be defined with @"Hello, World!" value.